Field of the Invention
Built-up bituminous roofing, commonly known as "tar and gravel" roofing, has been used successfully for many years. This type of roofing has enjoyed widespread use because of several factors: its relatively low cost, ability to prevent the spread of fire compared with wood shingle roofing, long life, and ease of repair. Thus, this type of roofing has been used extensively on both residential and commercial structures.
The construction of built-up roofing has been more or less standardized in the United States for at least the last 40 years. A typical roofing procedure is as follows: Base sheets of lightweight sheathing paper (e.g., 15 lbs./100 sq. ft.) are cemented in overlapping fashion with asphalt or tar to the roof deck, which may be of wood, gypsum, concrete, etc. A cementing layer of molten asphalt or molten tar is then applied by mopping or rolling it evenly over the sheathing paper at a typical rate of about 25 lbs./ 100 sq. ft. A layer of asphalt- or tar-saturated roofing felt is then placed over the cementing layer, overlapped, and (with steep-pitched roofs), nailed to the roof base. Another coat of cementing bitumen is then applied over the felt, and alternate layers of felt and cementing bitumen are applied in the manner described above until a total of 3 to 5 or more felts are in place. A final heavy layer of bitumen, called a flood coat, is applied over the top layer of felt at a rate of about 60 to 100 lbs./100 sq. ft. A layer of mineral surfacing material is then placed on the flood coat. This material is usually gravel, slag, or crushed stone, and is ordinarily applied at a rate of from about 300 to 500 lbs./100 sq. ft.
A recent development in built-up roofing has been the use of precoated asphalt felts with a consequent modification of the roofing construction. These felts which, in addition to being asphalt-saturated, are coated at the factory on both sides with asphalt which may contain a mineral filler. They are used in roofing systems which employ but two felts, yet are comparable in service life to conventional roofs constructed with 3 to 5 asphalt-saturated felts. The construction techniques employ the same sheathing layer, first cementing coat, flood coat, and mineral surface as used in forming the conventional roofs, but substitute the two precoated felts bonded with one cementing coat for the 3 to 5 saturated felts and their cementing layers.
Many advantages inhere in the use of roofing systems using precoated felts. Probably foremost is the reduction in labor costs resulting from the necessity of applying only two coated layers, rather than the three to five as in ordinary construction techniques. Other advantages which result from use of the coated sheets are enhanced resistance to water and mechanical damage of the sheets during construction and the greatly increased uniformity of the asphalt coating on each sheet, eliminating the possibility of air pocket formation, etc.
While for most applications and in most parts of the world where the precoated felt has been used, its performance has been highly satisfactory, in certain geographic areas, problems of roof slipping have resulted. In these cases, where the roof is pitched, a complete layer of the coated felt (usually the top one), topped with flood coat and gravel, will often separate from the first felt and be displaced a significant distance. This generally necessitates complete roof replacement. The problem has usually arisen in those areas where elevated summer heat caused a sufficient rise in temperature within the roof laminate to reduce the viscosity of the cementing layers of asphalt and allow flow and breakage of the cementing bonds.
My copending application, U.S. Ser. No. 812,315, filed Apr. 1, 1969, teaches one method for reducing the tendency of built-up roofs to slide. In that application, sand particles or the like are placed between the felt layers to inhibit sliding. This subject invention is directed to an alternative method for inhibiting sliding.
From my studies of sliding roofs, the conclusion has been reached that the sliding rate of roofs made with coated felts, when hot mopped asphalt is used as the binder, is proportional to the film thickness of the mopping asphalt. As the thickness of the mopped asphalt is reduced, the sliding rate correspondingly is reduced. And, since the hot mopped asphalt acts primarily as a binder of the felts (it is not needed for waterproofing the roof), the thinner the coat of asphalt which can be formed between the felt layers, the more economical the roof and the less the chances of failure of the roof structure.
Since the application of thin films of hot mopped asphalt is a practical impossibility, the use of another form of binder is necessary. This invention then is directed to the use of asphalt emulsions to bind the roofing felts, since they have the ability to form thin films. However, as will become evident from the specific examples set forth hereafter, only certain types of asphalt emulsions are satisfactory. The initial adhesion and tack of the emulsion bonded systems must be sufficiently high to (1) stay in place without sliding out of position and to (2) stay in place when the wind attempts to get under the sheets and peel them off. Simple bituminous emulsions bind the felt layers by slight softening of the layers being bonded and diffusion of the emulsion residuum into the asphalt coating of the felt layers to form a cohesive unitary mass. This process does not proceed fast enough to give the good initial adhesion strength required. This invention is directed to a method for making bituminous emulsions satisfactory binding agents for built-up roofs.